When tires become worn, they may be restored with new tread bands or tread layers during a retread process. Retreading is a restoration or re-manufacturing process that not only extends the service life of the tires, but also is significantly less expensive than manufacturing new tires. Since recycling and retreading are key for reducing costs and energy inherent in the manufacturing of tire casings, an effective retread necessitates a tire casing with good structural integrity (i.e., without internal anomalies or irregularities).
Before replacing the tread, it may be advantageous to inspect the tire, including the reinforcement elements of the body ply, for damage or wear. In certain situations, inspection may reveal that replacement of the tire is required rather than retreading. Alternatively, repair of the tire may be required since not all damage to interior elements (e.g., the reinforcement elements of the body ply) is readily apparent from a visual inspection alone.
Prior and/or subsequent to retreading of a truck tire casing, one or more non-destructive testing (NDT) methods may be used to detect and locate internal anomalies. Such anomalies may include, but are not limited to, cracks, voids, delaminated layers and/or foreign material. Numerous attempts have been made using advanced NDT techniques, and several types of inspection procedures have been employed and commercialized by the tire remanufacturing industry.
As the reinforcement elements for commercial tires such as heavy truck tires are frequently constructed from a ferrous material, one or more sensors can be used to detect discontinuities in the reinforcement elements. In an exemplary configuration, an apparatus for detecting anomalies in a tire metallic cable may include a plurality of magnetic field sensors positioned along a common line and configured to produce individual electrical signals proportional to a sensed magnetic field. A magnet having north and south poles can be positioned to provide a magnetic field at each sensor parallel to the common line. The alignment of sensors and the magnet may be such that the flux lines from the magnet are generally parallel to the plane occupied by the sensors. A tire cable anomaly present between the sensors produces a detectable difference in signals produced thereby as a result of formation of perpendicular flux patterns produced by the anomaly. Such an apparatus and an exemplary method of use thereof is disclosed by co-owned and co-pending U.S. Ser. No. 13/260,744 for TIRE METALLIC CABLE ANOMALY DETECTION METHOD AND APPARATUS, filed 31 Mar. 2010, the entire disclosure of which is incorporated by reference.
Such sensor systems detect small amounts of magnetic flux leakage from a loss of cross sectional area of metallic tissue when such tissue is placed within the static magnetic flux field of the sensor system. The amount of flux leakage may be extremely small (e.g., on the order of 5 to 10 gauss), yet it must be detected within a static field of several hundred gauss. Several factors in the makeup of the sensor can influence the sensitivity of detection. For instance, there may be variability in the strength of the permanent magnets due at least in part to manufacturing differences. A reduction of strength over time may be due to shock, or there may be variability in the magnet joint. There may also be variability of the magnet's position with relation to a sensor array and/or variability of the sensor array position with relation to a contact cover of the sensor system. There may also be variability inherent in an individual flux sensor's sensitivity and linearity. It is also possible that the magnetic poles could be reversed in relation to the plurality of magnetic field sensors positioned along a common line. These and other variables make it necessary to ensure that each sensor system is constructed to within some tolerance of uniformity.
While there are commercially available gauss meters to measure flux density, such devices only address the issues of magnet strength and position. These devices disregard the sensitivity and position of each flux sensor. Such devices may also introduce inherent difficulty in repeating placement of the sensor such that two or more sensor systems may be compared to one another.
Therefore, reliable and cost-effective quality indicators for a flux leakage detection system are demanded that ensure repeatable and predictable positioning during data collection.